The invention provides a method and a device for controlling the power-on flow of a multi-
branch battery energy storage
system. The method mainly comprises the following steps: an MBCU obtains the
voltage of each
branch according to state information reported by the S-BCU of each
branch, the MBCU enable the power-on flow of partial branches of which the voltages exceed the average
voltage of the various branches upon judging the imbalance of the voltages of the various branches and when a charging / discharging request is external discharging, and when the charging / discharging request is external charging, the MBCU enables the power-on flow of partial branches of which the voltages are lower than the average
voltage of the various branches. According to the method, when the voltages of the branches are in an imbalance state, partial unbalanced branches are enabled to enter the power-on flow firstly by use of the MBCU, and each branch is powered on in different time so that the
system stability can be improved. As a plurality of branches connected in parallel, the branches out of order can be flexibly removed without affecting the work of other branches, and therefore, the reliability of the
battery energy storage
system is improved.